The only problem is that I could not find say "user.home" anywhere on my hdd.
That's because it is a java property key. Run the Test app and see what the output is — it will show you the address of your home folder.

Can you expand on why this trick is needed?
Look up the put method in the Properties class api:
You find it down in the section Methods inherited from class java.util.Hashtable. Follow the put link:
The Hashtable put method takes an Object; in this app a String.DEFAULT_WIDTH is a primitive data type int.
If you didn't use the trick to make a String you would do